What is the difference between synthetic, semi-synthetic, and mineral engine oil?

What Is a Mineral Oil?

Mineral oils have been around since petrol and diesel engines were invented. You needed something to lubricate and help dissipate heat from moving metal parts as well as protect against corrosion inside the engine. Various versions of mineral or regular oils have done this job effectively for many years. It has only been the push to improve engine efficiency to give better fuel economy that has brought about the changes to lubricating oil. The sky rocketing price of fuel is the smoking gun or catalyst for the change.

Synthetic oils are now the norm in new vehicle engines and even slightly older engines that used mineral oil are changing to semi-synthetics. It is getting harder to find mineral oils on the shelf these days. 

Mineral oil is refined from crude oil which oil companies pump out of the ground. It still contains many additives to help the oil keep your engine running longer.

This includes things like:

• Rust & Corrosion Inhibitors – Stops rust and corrosion forming.
• Antioxidants – Stops build-up of sludge and acids.
• Anti-wear Agents – Adds a protective film to moving parts.
• Detergents & Dispersants - Clean the engine and suspend harmful particles.
• Friction Modifiers – Reduce the friction co-efficient between surfaces.
• Anti-foaming Agent - Stops oil frothing.

These are not all the additives included in mineral oil, but it helps to explain that it is not simply a base oil you are using in your engine. Quite a bit of chemical engineering makes up a modern mineral oil including being multigrade, performing in hot and cold environments.

What Is a Semi-Synthetic Oil?

Semi-synthetic oils or synthetic blend oil is, as the name suggests, a blend of mineral and synthetic oil. They are like a hybrid and offer advantages over mineral or conventional oils, though do not have the overall advantage of a full synthetic oil. They are a good in between step if using a mineral oil and want to get some of the benefits of a synthetic oil.

Just make sure to keep the viscosities to manufacturers’ recommendations as engine tolerances will be made to suit them. You also want to replace all the engine oil when doing this, don’t just top up with the semi-synthetic. 

What Is a Full Synthetic Oil?

Full synthetic oils have been around a lot longer than most people realise. Mainly used in full competition engines and aircraft engines, synthetic oil was already in use in the 1920s and ‘30s in limited quantities. During World War 2, when crude oil became scarce in Germany, they pushed synthetic oil to new levels. It started to make in-roads in production vehicle engines in the late ‘70s after the oil crisis sent fuel prices to unheard of levels.

Being thinner than conventional oil it offers less resistance or friction to rotating metal parts — which an engine has plenty of. If the engine spins more freely, it is using less power during acceleration and holding a particular speed on the road, therefore saving fuel. This is the primary reason for its adoption.

There are other advantages, such as:

• More stable than conventional oils so will degrade less quickly. That means servicing intervals can be extended; while the oil costs more, it is changed less often.
• Viscosity stays more consistent in cold and hot conditions compared to a mineral oil.
• They still have additives found in a regular oil.
• They can withstand the high heat produced in turbocharged engines better than a mineral oil.

Synthetic oils are still petroleum-based and developed from crude oil. It is not the same as a mineral oil. They break it down rather than refining it and take the parts they need. How they do this exactly is still a well-kept secret among manufacturers.

For synthetic, semi-synthetic, or mineral oil, shop online with Repco or head into your nearest store to get help with your purchase from a friendly staff member.